Radio frequency (RF) power amplifiers, which are used to amplify RF signals to obtain desired transmitter output power, are necessary components of most RF communications equipment. As is well known, RF power amplifier stages must be provided with a DC (direct current) bias voltage during normal operation in order to establish an operating point. Because application of bias voltage to an RF amplifier stage in the absence of an RF input signal often causes instability, bias voltage is normally applied only when it is desired to transmit an information signal. Thus, switching circuitry responsive to a transmitter control signal is generally required. The need for this control circuitry and signal lines for the distribution of the transmitter control signal adds complexity and cost to RF amplifier designs.
RF amplifier stages can also demonstrate instability when a high level of bias voltage is switched to the RF amplifier stage before the RF input signal rises to its final value. Allowing the bias voltage to be controlled or driven by the RF input signal leads to greater RF amplifier stability, particularly at low input signal levels.
Accordingly, a need arises for an RF amplifier biasing technique that limits parts count and complexity while helping to ensure amplifier stability.